U.S. Pat. No. 4,938,563 owned by Minnesota Mining and Manufacturing Company, describes a method of manufacturing a retroflective sheet material. One side of a transparent polymer plastic film is embossed with an array of reflecting elements. The composite of reflective elements and transparent film is then backed by a layer of material to strengthen the composite and protect the reflective elements from dirt or moisture. The opposite side of the transparent polymer plastic film, the side which is not embossed, serves as a transparent top film for the reflective elements.
The Minnesota Mining and Manufacturing Company patent describes deficiencies in retroreflective sheet material. In the past the cone of reflected light from the reflective elements was too narrow for many uses. When an effort was made to spread the light to increase the observation angle from which the retroreflective sheet material could be seen, much of the spread light was wasted leaving the retroreflective brightness of the article significantly reduced. The resulting retroreflective sheet material had diminished retroreflective brightness while still having a limited observation angle.